Jason and Médée, pursued by the people of Thessaly because of Médée's crimes, have sought refuge in Corinth. Médée is worried that Jason is growing distant from her. Jason claims he needs to win the good graces of the princess Créuse so her doting father, King Créon, will protect them. He suggests that Médée should give Créuse a beautiful robe as a present. After Médée leaves, Jason confides that he is really in love with Créuse but fears Médée's reaction. Créuse is due to be married to Oronte, prince of Argos, who now arrives in Corinth with his army. However, King Créon tells Jason that he would prefer him as a son-in-law. Jason leads the combined Corinthian and Argive army to victory against the Thessalians.
Créon tells Médée he will not hand her over to her enemies but she must leave Corinth. Jason and his children by her will stay. Médée protests that she only committed those crimes out of love for Jason, but Créon replies that the Corinthian people want her to leave. Médée hands over her children to Créuse. Créuse confesses her love to Jason.
Oronte promises Médée refuge in Argos if she can arrange a marriage between him and Créuse. She tells him that the only reason she is being banished is so Jason can be free to marry Créuse. They must combine forces to prevent this happening. Jason pleads with Médée that he is only acting in the best interests of their children. Left alone, Médée resorts to witchcraft and summons demons from the underworld who bring her a poisoned robe for Créuse.
Jason admires the beauty of Créuse's new robe. Oronte finally realises that what Médée had said is true: Créuse will marry Jason, not him. Médée vows that Créuse will never be Jason's bride. Créon arrives and is angered that Médée has not yet left Corinth. He orders his guards to seize her but she conjures up spirits of beautiful women who seduce the guards away. Then she uses her magic powers to drive the king insane.
Médée rejoices at her success and plans to take her vengeance to an extreme by murdering her own children by Jason. Créuse begs her to spare Corinth, even pledging to renounce her wedding to Jason if she does so. News arrives of Créon's madness and death. Médée touches Créuse's poisoned robe with her wand and it bursts into flame. Créuse dies in Jason's arms. Jason swears revenge on Médée, who now appears in a flying chariot pulled by dragons to announce she has stabbed their children. She leaves as the palace of Corinth bursts into flames.